"I'm here to tell you it is well worth a read. Simple and quick and yet so rewarding. As Donna Tartt says in her great introduction – it's all too easy to say that we "love a book" based on one read. However, I've felt the thunderbolt and I am ready to love. I know it's always going to be in my life and the lives of my family to read again and again and to introduce to new generations young and old."

So my first question is, how has it struck you?

When I first read it I'd already felt that thunderbolt – but I enjoyed it even more the second time. I still found the story gripping – but knowing the resolution made it easier for me to relax and focus on a few other things. Most particularly, Mattie's voice.

Reading group contributor pogwilson described her voice as "pitch perfect", a sentiment echoed by writer George Pelecanos, no less, who told America's NPR, "I don't think anyone has come close to what Portis achieved with this book. It's all in the voice of Mattie Ross."

It's there from the very first words in the book:

"People do not give it credence that a 14-year-old girl could leave home and go off in the wintertime to avenge her father's blood but it did not seem so strange then, although I will say it did not happen every day."

And it continues just as strong from then on; a masterpiece of straight-backed deadpan. Mattie describes an unsatisfactory meal at a boarding house and scenes of horrific violence with the same blend of almost scientific fascination and emotional neutrality. She might mock journalists throughout the book (and might have failed in her own attempts to get her articles into "the magazines of today" who prefer to print "trash"), but the blend of dialogue and first-person observation is as good as reportage gets.

In the boarding house, we see "a tall, longbacked man with a doorknob head and a mouthful of prominent teeth," business talk about "pocket calculators" (whatever they may be) and men "occupied with their food like hogs rooting in a bucket." But the food is not good. "Watch out for the chicken and dumplings," someone tells Mattie. "They will hurt your eyes." Why? "They will hurt your eyes looking for the chicken." Mattie, always the penny-pinching pragmatist, and never afraid to offer her opinion, sums up the meal thus: "The dumplings were all right but I could not see twenty five cents in some flour and grease."

She offers just the same kind of conclusion after watching the grim demise of the bandit, Moon. This time the proceeding dialogue is almost as clinical as Mattie's summation. Rooster, the man she's hired to avenge her father, coolly tells the dying man, "Your pard has killed you and I have done for him". The description, too, is sharp and brief. We see the man trying to grasp a cup of water with his "bloody stump" and talking, "in a rambling manner and to no sensible purpose." Then he goes quiet and Mattie tells us: "Here is what was in his eyes: confusion." If that isn't stark enough she adds: "Soon it was all up with him and he joined his friend in death. He looked about thirty pounds lighter."

It's vivid, and convincingly sincere – and yet also hilarious. Mattie's Old Testament, eye-for-an-eye approach to violence, fierce pragmatism ("If you want anything done right you will have to see to it for yourself every time") and determination to talk straight, all leave her just a few beats off the normal rhythms. We can't help but laugh at her – even as we respect and admire her, and even though Portis also allows her the dignity of getting in a few good lines herself. "I would not put a thief in my mouth to steal my brains," she tells Rooster when he is drinking.

Mattie's voice isn't just impressive as a lesson in vivid description and comedy. Throughout the book, she's entirely convincing both as a young girl, and as the old woman remembering that girl (and quite possibly transferring her own starchy Presbyterian conservatism on to her younger self). A few choice commentaries on how the places Mattie visits have changed, and observations about (for instance) motor cars, snap us out of the old west just long enough to make us realise that Mattie's youth, and an entire way of life, have disappeared. "Time gets away from us," Mattie says at the end of the book. I defy you not to feel a lump in your throat when you get there.

"Portis destroys absurdity by overwhelming it with truth. Mattie Ross is 19th-century America; it is impossible to doubt her, impossible to doubt the tale that she tells. Hers is a yarn with swagger, colour and song."

Mattie's voice is so convincing that it seems astonishing that it might have been created by a man in 1968 rather than the real 19th-century woman.

But then again, if you do disagree with Garfield, your reading may be a particularly good one. Is it really impossible to doubt Mattie? How much credence should we honestly give her "true account of how I avenged Frank Ross's blood over in the Choctaw Nation"? She may chide others for "stretching the blanket" – but every now and again we get a sense that she may be doing that very thing. The doubtful reactions of her contemporaries in the 20th century, her own suggestion that Rooster's stories are "hard to believe" and other scattered hints do just enough to open up the interesting possibility that her incredible adventure may actually be a tall tale.

And there's yet another layer to this surprisingly complex narrative. The real author was a Korean war veteran, publishing during Vietnam and just after the summer of love. The historical context wasn't something that occurred to me first time through, but this time around, I began to wonder. What for instance, are we to make of Mattie's, "Thank God for the Harrison Narcotics Law"? Not a fan of The Beatles? "Also the Volstead act." What are we to make of the slaughter in the book? What do Mattie's thoughts on Indians and entering their territory tell us about the American dream and the civil rights struggle? Perhaps nothing. Perhaps a great deal. Like all great novels, True Grit is open to interpretation. Which is a good point to hand over to you.

Or almost! One quick announcement. We're going to be discussing the Hollywood adaptations of the films later this month and before we do, I'm happy to say we have 10 copies of True Grit (out now on DVD and Blu-ray) to give away.

The first 10 people from the UK to post an "I want" in the comments below will get one. Although, don't forget to email in to ginny.hooker@guardian.co.uk afterwards, letting us know your address and your user name. We can't track you down ourselves!